In the description which follows, reference is made primarily to the eye as the object. This has to be understood as merely a way to help the description and not as a restriction of the application of the present invention. As such, where the term “eye” is used, a more general transparent and scattering object or organ may be sought instead. In the case of the eye, the object is the retina which is to be imaged via the anterior chamber which introduces aberrations. When a specimen in microscopy is the object, the aberrations are introduced by the microscope objective, the microscope slide or other intermediate plates and optics devices, or even by the superficial layers of the specimen.
Low coherence interferometry is an absolute measurement technique which allows high resolution ranging and characterisation of optoelectronic components as presented in the papers S. A. Al-Chalabi, B. Culshaw and D. E. N. Davies, “Partially coherent sources in interferometric sensors”, First International Conference on Optical Fibre sensors, 26-28 Apr. 1983, I. E. E. London, pp. 132-135, 1983, R. C. Youngquist, S. Carr, and D. E. N. Davies, “Optical coherence-domain reflectometry: A new optical evaluation technique,” Opt. Lett. 12(3), pp. 158-160 1987 and H. H. Gilgen, R. P. Novak, R. P. Salathe, W. Hodel, P. Beaud, Submillimeter optical reflectometry”, Lightwave Technol., Vol. 7, No. 8, pp. 1225-1233, 1989.
The first application in the biomedical optics field was for the measurement of the eye length as shown in A. F. Fercher, K. Mengedoht and W. Werner, “Eye length measurement by interferometry with partially coherent light”, Opt. Lett., Vol. 13, No. 3, (1988), pp. 186-189.
Adding lateral scanning to the scanning in depth, allows acquisition of 3D information from the volume of biological media. This concept, of adding devices for lateral scanning in an interferometer, has been presented in papers on heterodyne scanning microscopy, such as “Optical heterodyne scanning microscope”, published by T. Sawatari in Applied Optics, Vol. 12, No. 11, (1973), pp. 2766-2772.
The potential of the technique for high resolution imaging of the tissue is often referred to as optical coherence tomography (OCT) as presented in D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, ‘Optical coherence tomography’, Science 254, pp. 1178-1181, 1991 and in the paper “Optical coherence tomography” by A. F. Fercher, in J. Biomed. Opt., 1(2), (1996), pp. 157-173. OCT has the potential of achieving high depth resolution, which is determined by the coherence length of the source. For example, optical sources, such as superluminiscent diodes and mode-locked lasers are now available with coherence lengths below 20 μm.
An OCT apparatus is now commercially available, which produces longitudinal images only, i.e. images in the planes (x,z) or (y,z), where the z axis is perpendicular to the patient's face and x and y axes are in the plane of the patient's face. Examples of such apparatus for longitudinal imaging are described in U.S. Pat. Nos. 5,493,109, 5,537,162, 5,491,524, 5,469,261, 5,321,501 and 5,459,570.
In the documents and patents above, A-scans, which are axial reflectivity profiles are generated, and B-scan images are obtained by grouping together several A-scans for adjacent transverse position of the scanning beam. Different scanning procedures are explained in the patent application US20030199769A1.
OCT has also been reported as being capable of providing en-face, or transversal profiles, or T-scans, which are reflectivity profiles generated by moving the beam transversally across the target. Based on T-scans, constant depth images (C-scan, or images with the same orientation as in microscopy) can be generated, as reported in “Coherence Imaging by Use of a Newton Rings Sampling Function” by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., Vol. 21, No. 21, (1996), pp. 1789-1791, “Simultaneous En-face Imaging of Two Layers in Human Retina” Opt. Letters, by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., 1997, vol. 22, No. 13, pp. pp. 1039-1041, “En-face Coherence Imaging Using Galvanometer Scanner Modulation” by A. Gh. Podoleanu, G. M. Dobre, D. A. Jackson, Opt. Lett. 23, pp. 147-149, 1998 and in “Transversal and Longitudinal Images from the Retina of the Living Eye Using Low Coherence Reflectometry”, by A. Gh. Podoleanu, Mauritius Seeger, George M. Dobre, David J. Webb, David A. Jackson and F. Fitzke, published in the Journal of Biomedical Optics, 3(1), pp. 12-20, 1998. T-scan technology is also described in the U.S. Pat. No. 5,975,697.
The OCT technique applied to ophthalmology has evolved rapidly in the last few years, as it can deliver a much better depth resolution than a scanning laser ophthalmoscope (SLO), based on the confocal microscopy principle. The technology of SLO was presented in R. H. Webb, G. W. Hughes and F. C. Delori, “Confocal scanning laser ophthalmoscope,” Applied Optics, 26, 1492-1499 (1987). SLOs deliver C-scan images.
The limitations of the longitudinal OCT imaging instruments have been addressed in two respects: (i) establishing procedures to generate en-face OCT images from the retina, as mentioned in the papers by Podoleanu mentioned above, and (ii) design of a dual channel OCT/confocal instrument for the eye, as disclosed in the U.S. Pat. No. 5,975,697.
However, important limitations still exist in imaging with high resolution the retina and tissue in histology. The transversal resolution in OCT imaging is governed by the optics of the eye and its aberrations. Adaptive optics was employed in a flood illuminated eye as described in US2003/0025874A1 and in J. Liang and D. R. Williams”, Aberrations and retinal image quality of the normal human eye”, JOSA A 14: (11) 2873-2883, (1997) and improved the transversal resolution to the point where it was possible to distinguish the cones in the fovea as shown in J. Liang, D. R. Williams, D. T. Miller, “Supernormal vision and high-resolution retinal imaging through adaptive optics”, JOSA A 14: (11) 2884-2892, (1997). AO utilises two devices operating in closed loop. In the AO system, a wavefront sensor measures the aberrations by evaluating the phase distribution over a defined plane. This information is used to actuate a wavefront corrector to imprint distortions of opposite sign in order to convey an aberration free image to the display. A flying spot ophthalmoscope, (i.e. an ophthalmoscope using scanning the beam point by point and producing a display signal for each pixel where the spot is incident, in opposition to a fundus camera which flood illuminate the retina, incorporating AO elements was reported by A. Roorda, F. Romero-Borja, W. J. Donnelly III, H. Queener, T. J. Herbert and M. C. W. Campbell, “Adaptive optics scanning laser ophthalmoscopy”, in Opt. Express, Vol. 10, No. 9, pp. 405-412, (2002) which achieved a resolutions of 2.5 μm transversal and 100 μm axial in the eye.
Adaptive optics was also reported in being used to compensate for the aberration in microscopy. Such a possibility is described in “Smart microscope: an adaptive optics learning system for aberration correction in multiphoton microscopy”, by G. Albert et al, published in Opt. Letters, vol. 25, No. 1, January 2000, pp. 52, 54. Use of AO has lead to an increase of the image size by 9 times due to extending the useable areas of focusing elements close to the edges.
As mentioned above, OCT provides means for achieving a high resolution in depth for optical systems of low numerical aperture (NA). AO provides means to correct for the aberrations in the optical path and in this way, to improve both transverse and depth resolution, to the level allowed by the NA of the interface optics.
If solutions are found to combine the two technologies, OCT and AO, then high resolution could be achieved both in lateral and in depth directions. In this way the minimum resolved volume, the voxel, could be reduced.
The patent application WO2003/105678 A2 discloses a system where a wavefront corrector is used to compensate for the aberrations of the eye in a flood illuminated system incorporating an OCT channel. As a disadvantage of the system disclosed, both reference beam and object beam from the interferometer traverse the wavefront corrector. The reference beam is free of aberrations and there is no need to correct it, and in fact it will be practically aberrated by the corrector. Placing the wavefront corrector after the interferometer, the corrector sees two optical signals, object and reference. Both object and reference beams are routed via the wavefront corrector with disadvantages in terms of system complexity, and reduction in the corrector efficiency, as central actuators in the correctors are sacrificed to reflect the reference beam.
As another disadvantage, the OCT system in WO 2003/105678 operates as a full field time domain OCT, or coherence radar, where the 2D interference map of a reference local beam and an object beam returned from the object is displayed by a 2D CCD camera, which produces C-scan OCT images. It is known that flood illumination imaging of the fundus is inferior to flying spot in terms of signal to noise ratio. Flood illumination relies on the dynamic range of CCDs, which is maximum 16 bits. Coherence radar or full field OCT systems (as that in WO2003/105678 A2) can measure reflectivity not smaller than 10−5. The signal to noise ratio is in this way smaller than that possible to be achieved in flying spot OCT, which can in principle measure 10−14 reflectivity.
As another disadvantage, B-scan OCT images can only be produced after a stack of C-scan images have been collected from different depths, and by software means, a B-scan is inferred from the 3D volume of data, i.e. a B-scan cannot be produced in real time.
The paper Adaptive optics parallel spectral domain optical coherence tomography for imaging the living retina by Yan Zhang, Jungtae Rha, Ravi S. Jonnal, and Donald T. Miller, published in Opt. Express, Vol. 13, No. 12, Page 4792-4811 presents a combination of an AO system with a spectral domain OCT (SD-OCT) camera based on a free-space parallel illumination architecture. Again, the AO correction operates in the emergent beam only, and as in the system of WO 2003/105678, on both reference and object beams.
As another disadvantage of the Zhang system, the two images generated in sequential regime cannot be compared to each other. C-scan fundus flood illuminated images and B-scan OCT images are obtained sequentially. By removing the diffraction grating and replacing it with a mirror, the CCD in the system is used to read a C-scan image in the fundus camera regime instead of a dispersed optical spectrum in the spectral OCT regime. Because in one regime, fundus camera, a C-scan is generated while in the OCT regime, a B-scan image, due to their rectangular orientation, the two images are not compatible.
As another disadvantage of the Zhang system, the fundus camera regime demonstrates improvement in the transverse resolution of C-scan images when AO is applied, but because no confocal aperture is used, the depth resolution is larger than that achievable in a genuine confocal system.
Another disadvantage of the flood illumination used in the system and the system of WO 2003/105678, is that the rays enter and return from the eye, or from the microscope objective in microscopy, within a fan with an angular extension given by the size of the lateral image size on the retina, or on the specimen in microscopy respectively. These rays “see” different aberrations and the AO system can only compensate for an average of cumulated aberrations over the angular extension of the fan of rays. Therefore, a flood illuminated AO system requires a separate source to provide the optical beam for the wavefront sensor.
Also, flood illumination means that some scattering from adjacent points to a pixel reach the photodetector in the CCD array corresponding to that pixel generating cross talk and noise.
The paper Adaptive-optics ultrahigh-resolution optical coherence tomography, by B. Hermann, et al, published in Optics Letters, Vol. 28, No. 18, 2004, 2142-2144, presents a slow flying spot system OCT where the fast scanning direction is in depth. Using A-scans, B-scan images are generated. The systems disclosed in the paper by Zhang and Hermann above are based on A-scans. Therefore, such systems cannot build a C-scan image in real time. Such C-scan can only be produced after several B-scans are acquired and then by software means, C-scans are inferred. The orientation of B-scan images is rectangular to that of confocal microscopy images, which provides C-scan images. C-scan images are desirable, because they are familiar to ophthalmologists, as scanning laser ophthalmoscopes are being used from 1981. C-scan images are also familiar to the microscopy community, as their real time output is a raster image oriented in a constant depth plane. C-scan images have the same orientation as that of fundus cameras or microscopes and are easier to interpret than cross section, B-scan images. Therefore is disadvantageous not to be able to generate real time C-scan images, especially in cases of moving organs and fast processes in biology, where the C-scan inferred from a 3D data volume acquired over time is corrupted by movement.
As an additional disadvantage of such systems, a large depth of focus is required when collecting A-scans. A depth of focus comparable with the depth range, 1-2 mm when scanning the retina. If AO is applied, the confocal core of the OCT could ideally shrink to less than 100 μm, therefore A-scan will be modulated by the focus profile with maximum sensitivity within a 100 μm range and insignificant values outside.
As another disadvantage, the Zhang's and Hermann's papers above refer to a regime, where OCT B-scan images are obtained under a static mirror configuration. The aberrations are read, correction evaluated, memorised and then the AO loop is opened. This does not allow correction of aberrations in real time.
All documents above refer to improvements in either the OCT channel (WO 2003/105678, Hermann) or the confocal channel (Roorda) with no attention given to correspondence of images between the two configurations and the scanning possibilities are limited due to the specific embodiments proposed. Hermann's Optics Letters paper is a flying spot system, based on A-scan OCT profiles only, which excludes real time T-scans and real time C-scans. Roorda's paper refers to a flying spot system which produces a raster imaging, a C-scan system only, in an SLO, which cannot achieve B-scan images in real time. System of WO 2003/105678 is not compatible with OCT B-scan imaging, as the interface optics, the lenses between the source and the eye need to be changed to project a line instead of a raster on the eye.
Such configurations cannot provide pairs of OCT and confocal images at the same time.
U.S. Pat. No. 5,975,697 shows how based on T-scans, pairs of OCT and confocal images could be generated at the same time, in two regimes, B and C-scan. However, because the resolution is limited in the confocal channel, one image in the pair, the confocal, does not provide any depth resolution in the either B or C-scan regimes.
Therefore it is desirable to provide solutions for the problems listed above.